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Dissociation Constants of Some Substituted Nitrophenols in Aqueous Solution at 25 Degrees C

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Dissociation Constants of Some Substituted Nitrophenols in Aqueous Solution at 25 Degrees C JOU RNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry Vol. 71 A, No.5, September-October 1967 . Dissociation Constants of Some Substituted Nitrophenols In Aqueous Solution at 2S 0 C R. A. Robinson Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (May 9, 1967) The dissociation constants of twelve substituted phenols with a nitro group in the 0- or p-position have been determined by spectrophotometric measurements in aqueous solution at 25°C. Key Words: Dissociation constant, nitrophenols, substituted phenols. 1. Introduction 2,4-Dinitro-6-methylphenol was recrystalli2;p.d twice from benzene-cyclohexane mp 86.0-86.7 °C; 2,6-dini­ The determination of the dissociation constants of tro-4-methylphenol from benzene-cyclohexane and tnen a number of substituted phenols by the spectrophoto­ from cyclohexane, mp 80.7- 81.2 °C; 2,4-dinitro-3- metric method has been described recently [1, 2, 3, 4, methyl-6-isopropylphenol from aqueous ethanol and 5, 6, 7).1 The purpose of this paper is to present data th en from li groin , mp 55.0-55_7 °C. 2,4-Dinitro-6- for twelve substituted phenols with an indicator phenyl phenol recrystallized from methyl ethyl ketone range pH 2.1-5.5. and th en from ethyl acetate, mp 206.6-207.0 °C; 2,4-dinitro-6-isopropylphenol from 95 percent ethanol and then from hexane, mp 54.0-55.0 0c. 2. Experimental Procedure Table 1 gives some characteristics of the absorption spectra of these compounds, recording the wavelengths 2.1 . Materials and extinction coeffi cients at maximum absorption in both acid and alkaline solution, and also those at the With the exception of 2,6-diiodo-4-nitrophenol, the isosbestic points. Figure 1 shows the absorption spec­ materials were commercial products. I am indebted trum of one of these phenols, 4-chloro-2,6-dinitro-3- to Dr. Marion M. Davis for the following details of methyl phenol, in alkaline solution where the phenol their purification. is present in the form of the phenolate anion, in 2-Chloro-4, 6-dinitrophenol was recrystallized twice strongly acid solution where it exists as the undissoci­ from benzene-cyclohexane, mp 111.0- 111.5 0c. 4- ated molecule, and in a dilute solution of hydro­ Chloro-2,6-dinitro-3-methylphenol was recrystallized chloric acid were both species are present; the isos­ once from benzene-cyclohexane and then from li­ bestic point is at 378 fl1I.Lwith an extinction coefficient groin, mp 68.0-68.5 °C; 2-chloro-6-nitrophenol first E=2,740. from 95 percent ethanol and then from benzene­ cyciohexane, mp 70.0-70.5 °C . 2,4-Dibromo-6-nitro­ TABLE 1. Characteristics of the absorption spectra of substituted phenol was purified in the same way, mp 117.0-117.5 phenoLs in aqueous soLution at 25 °C. 0c. 2,6-Dibromo-4-nitrophenol was recrystallized three times from benzene-cyclohexane, then from aqueous Alkaline Acid Isosbesti c ethanol and then from benzene-cyclohexane, mp solutio n solution poi nt E:2 tnJJ; IllM tnI' 141.8-142.0 °C, 2,6-dichloro-4-nitrophenol twi ce from .. • . benzene-cyclohexane, mp 125 °C (dec.). 2,6-Diiodo-4- 2-C hloro-4,6-din it rophe nol. _.. 14.910 370 10.100 268 4.250 326 4-C hloro.2,6.dinitro-3· me thylphenol .. 5.660 425 3,950 355 2.740 378 nitro phenol was synthesized by the method of Datta 2·Chloro·6·nitrophenol .. 4.580 420 2,620 350 2.200 372 and Prosad [8] by adding p-nitrophenol in an excess 2,4· Dibromo·6·n it rophenol .. .... , ... 4.1 80 430 2.370 365 2. 11 0 383 2,6· Dibl'omo·4·n it rophenol .... 15,720 402 7.650 312 3.900 344 " of ammonia to a solution of iodine in aqueous potas­ 2,6· Die hloro-4·n it rophenol .. 13,850 399 7.020 312 3.500 343 2,6· Diiodo-4-n it rophenol ..... .... .. ... 15.620 409 6.550 318 3.570 ·35 1 sium iodide, recrystallized from aqueous ethanol and 2,4- Dinitro·6·meth ylphenol. .. 14.2 10 377 13.630 'l.'72 5.330 333 2.6·Dinitro.4.merh ylphenol. .... 7.250 450 5.100 365 2.390 393 then from benzene-cyclohexane, mp 156.5-157.0 °C. 2,4-Dinitro·3·meth yl-6·isopropylphenol ... 14.630 396 5.500 300 3.900 335 2,4-Di nitro.6.phenylphenol .. 14.990 380 8,290 265 4.280 334 2,4-Din i tro·6-isopropyl phenol .. 13.400 375 5.680 265 5.220 335 I Figures in brackets indicale the literature references aJ the end of this paper. 385 TABLE 3. Dissociation constant of 4-chloro-2,6-dinitro-3-methyl­ 425 m}" phenol in aqueous solution at 2S °C 6,000 E2 =5,660 - a.Olm- NoOH ---- tm- Her 5,000 355m}" .......... 0.0012 m - Her Concentration of phenol: 2.6 X 10- 5 M, cell length 4 cm, wavelength . ,03,950 425 mJL , D , 0.012 , D2 0.583 4,000 /--, / , E / , 3,000 /1 ................. ~ mHCI X 10' D "'H + X 10' - 2 log Y pK / . / \~ ~8r~ }", . 02,740 2,000 "".. ;"' ... ....-/ ..... ', ....... .... \ \ 2.192 0.420 2.378 0.016 3.241 1,000 " 6.048 .293 6.176 .025 3.248 " 8.084 .257 8 .196 .029 3.239 300 350 400 450 500 550 10.28 .228 10.38 .032 3.232 WAVELE NGT H (m}") 12.06 .205 12.15 .034 3.241 FIGURE 1. Absorption spectra of 4-chloro·2,6-dinitro-3-methyl­ Average ......... .. ...................... ........... ... .. .. ...... , 3,240 phenol, 2.2. Method The spectrophotometric method of determining the dissociation constant of a weak acid has been described TABLE 4, Dissociation constant of2-chloro-6-nitrophenol in aqueous many times [1, 2, 3, 4, 5, 6, 7], For the five phenols solution at 2S °C with pK values less than 3_5, dilute solutions of hydro­ chloric acid were found to be suitable media in which to make measurements; for the other seven, with Concentration of phenol: 3 x 10- 5 M, cell length 4 cm, wavelength pK values greater than 3,8, succinate buffers were 430 mJL , D, 0,014, D2 0,538 Buffer solution: Sodium hydrogen succinate and disodium succinate, U!;;ed_ In some cases a small correction was needed each at molality m for the effect of the phenol on the acidity of the buffer solution; this correction has already been described [9], m D p(aHYcl) pK pK (co rr,) 3. Results and Discussion 0,0100 0,294 5,553 5,493 5,491 ,0175 ,290 5,532 5,485 5,483 Details of the spectrophotometric measurements ,0250 .285 5,511 5,481 5,480 are given in tables 2 to 13, The results are summarized ,0375 .281 5.499 5,482 5,481 in table 14. ,0500 .274 5,477 5,483 5,483 It is known that ortlto and para substitution of a nitro group in phenol have effects which are by no means Average ... ... .. ... .. , .. , .. , ....... .. .. ........ ................. 5.483 additive, Thus, if the pK value of phenol itself is 9_99 8 [10], that of o-nitrophenol 7,230 [11], and that of p-nitrophenol 7,156 [12], then 0- and p-nitro substitu­ tion lowers the pK \' alue , of phenol by 2,768 and 2_842, respectively, We might, therefore, expect pK = (9_998 - 2.768 - 2.842 )= 4,388 for 2,4-dinitrophenol, whereas TABLE 5. Dissociation con!tant of 2,4-dibromo-6-nitrophenol m aqueous solution at 2S °C TABLE 2, Dissociation constant 0/ 2-chloro·4,6-dinitrophenol Ln aqueous solution at 2S °C Concentration of phenol: 4.5 X 10- 5 M, cell length 4 cm, wavelength 430 mJL, D, 0.067, D2 0.757 Buffer solution: sodium hydrogen succinate and sodium chloride, Concentration of phenol: 1.18 X 10- 5 M, cell length 4 cm, wavelength each at molali t y m 372 mJL , D, 0.073, D, 0.660 m D p (aHYcl) pK pK (corr.) mHCI X 10" D mH+ X 103 -2 log Y pK 2.210 0.542 2.221 0.046 2.100 0.02 0.467 4.853 4.713 4.712 3.206 .508 3.216 .054 2.090 .04 ,461 4,833 4.709 4,708 4.967 .454 4.976 .065 2.101 .06 .458 4.821 4.704 4.704 7.389 .403 7.397 .081 2.103 .08 .454 4.811 4.705 4,705 10.05 .359 10.06 .089 2.108 .10 .449 4.802 4.709 4.709 Average .... ... ...... .. ......... .. .. ... ........ ...... .... ...... ' " 2.100 Average ...... ... ......... ............................. ......... , 4.708 386 TABLE 6. Dissociation constant of 2,6-dibromo-4-nitrophenol in TABLE 9. Dissociation constant of 2,4-dinitro-6-methylphenol in aqueous solution at 25°C aqueous solution at 25 °C 5 Concentrati on of phenol: 2.05 x 10- M, cell length 4 cm, wavelength Co ncentration of p.henol: 8.3 X 10- 5 M, cell length 1 cm, wavelength 402 mp.- , D, 0.005 , Dz 1.274 400 mp.-, D, 0.028, Dz 0.969 Buffer solution: sodium hydrogen succinate and sodium chloride each at molality m mHC' X 10' D II'H + X 10' - 2 log 'Y pK m D p(aH'YCI) pK pK (corr.) 2.013 0.855 2.150 0.01 5 3.376 3.106 .726 3.222 .018 3.391 4.018 .649 4.122 .020 3.392 0.02 0.691 4.853 4.476 4.471 4.980 .584 5.073 .023 3.394 .03 .688 4.840 4.469 4.466 6.057 .527 6.141 .025 3.393 .05 .680 4.826 4.473 4.471 6.999 .482 7.076 .027 3.397 .07 .675 4.816 4.474 4.473 8.015 .447 8.086 .028 3.392 .10 .669 4.802 4,472 4.471 9.003 .415 9.069 .030 3.393 9.996 .385 10.06 .03J 3.397 Average.
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